The present invention relates to a device for gripping and pulling a length of thermoplastic sheet softened to be moldingly formed at the front and rear edges. The edges which are otherwise inevitably made to be concave are held substantially straight.
Various articles such as bath tubs, refrigerator compartments and the like are formed by feeding a thermally softened plastic sheet having of a width and length necessary for forming the desired article between a pair of opposite dies separated with each other. Forming occurs by holding the opposite side edges thereof by means of a pair of endlessly driven cramp chains, and then engaging said upper and lower dies together.
The plastic sheet is in general continuously formed by a extruder having a slit nozzle of a desired gap and width, and cut in the necessary length. In either case where the cut sheets are directly fed from the extruder to a forming machine having dies or where the cut sheets once stored are successively supplied thereto e.g. from a pallet, they must be heated to be relevantly formed just in front of said dies.
The softened plastic sheet, which is seized by the pair of cramp chains at and along the opposite side edges to be fed to the forming machine, is inevitably dangled down in the longitudinal direction due to thermoplasticity and weight or gravity thereof, and shrinks more or less in the length thereof due to cooling during feed so that the originally straight front and rear edges of the sheet are made concave, which is more definitely illustrated later.
Not only does such dangling of the plastic sheet have adverse affects on relevant molding of the articles, but also such shrinkage in the lengthwise direction necessitates that the plastic sheet is cut longer so as to compensate for shrinkage, which results in material loss.
It is not always simple to grip the concave front and rear edges of the plastic sheet so as to suitably pull the edges to be substantially straight in the forming machine without disturbing the forming operation.
In JP-A Sho No. 62(1987)-161523, there is disclosed a device for gripping and pulling the concave front and rear edges of the softened plastic sheet comprising a pair of upper and lower clamp members adapted to move vertically apart for allowing the plastic sheet to be brought between the upper and lower dies and towards each other for holding the sheet at the rear edge therebetween with each other. Another pair of upper and lower clamp members are adapted to move vertically apart for allowing the article formed on the plastic sheet to be discharged out of the forming machine and towards each other for holding the sheet at the front edge therebetween with each other. Each of the two pairs of clamp members being mounted on a respective carriage adapted to longitudinally move for pulling the clamped edges rearwardly and forwardly.
This is not satisfactory, however, in that there must be provided two carriages to be longitudinally moved for pulling the softened plastic sheet in opposite directions in addition to the vertically movable clamp members, and independent therefrom so that it is difficult to actuate pulling timingly and with suitable force relative to clamping.